Seventy-five salt water beaches on the South Shore are tested for enterococci, intestinal bacteria found in humans and animals.

High levels of enterococci indicate the waters may also contain other disease-causing microbes that are present in sewage but are more difficult to detect. Bacterial colonies are filtered from three ounces of water and placed on a gel infused with nutrients and chemicals designed to promote growth.

Left in an incubator, the single cells isolated on the filter grow explosively, forming colonies visible to the naked eye.After one day, the colonies are counted and if they exceed 104 colonies, the beach is closed to swimming.
Source: Woods Hole Oceanographic Institution